In some designs of power inverter circuits, two power MOSFET switches are arranged in a totem pole (half-bridge) topology. The power MOSFET switches are driven to conduct alternately, in which one of the MOSFET switches is designated as a high side switch, and the other MOSFET switch is designated as the low side switch. By selectively switching the power MOSFET switches in an alternating fashion, a load can be driven with an alternating current, forming a DC to AC inverter.
The gate of the high side switch is typically driven by a bootstrapped power supply, and a bootstrap capacitor is used to increase the voltage available to the gate of the high side switch. A MOSFET is a voltage-controlled device which will allows very little gate current to pass, thereby making it possible to utilize the charge inside the capacitor for control purposes. However, due to parasitic gate current and non-ideal (i.e. finite) internal resistance the capacitor will eventually lose its charge. At very low (or steady-state) switching speeds, the bootstrap capacitor may discharge without having an opportunity to recharge. Most control schemes that use a bootstrap capacitor force the high side driver off for a minimum time to allow the capacitor to recharge.
Such bootstrapped architectures are generally operated at less than 100% duty cycle rather than a steady-state basis, sacrificing a margin of performance in order to allow for the capacitor to recharge. However, such circuits may still encounter situations in which external conditions may cause a 100% or near-100% operating condition, such as when a motor being driven by the inverter circuit becomes stalled.